Use cx_oracle.LONG_STRING /LONG_BINARY for CLOB/BLOB

Use cx_oracle.LONG_STRING /LONG_BINARY for CLOB/BLOB

Changed the implementation of fetching CLOB and BLOB objects to use
cx_Oracle's native implementation which fetches CLOB/BLOB objects inline
with other result columns, rather than performing a separate fetch. As
always, this can be disabled by setting auto_convert_lobs to False.

As part of this change, the behavior of a CLOB that was given a blank
string on INSERT now returns None on SELECT, which is now consistent with
that of VARCHAR on Oracle.

Fixes: #5314
Change-Id: I7b46c91704b6f5d6c157e083505dac6e0cb3ef6e